This invention relates to an apparatus and method for reproducing an image and, in particular, to an apparatus and method for reproducing an original image containing screened artwork.
In the production of the Yellow pages, hard copy master pages are prepared for use in the production process. These master pages include listings, line artwork and screened artwork. Listings are produced by a contrast sub-system and line and screened artwork by an Ad generation sub-system.
In line artwork, the images comprise continuous areas of either black or white and there is no gray level or tone present. In screened artwork, the images contain black dots (so-called "halftone dots") having a predetermined frequency (screen frequency) in dots per inch. These dots are changed in size from area to area to provide different shades of gray from black (i.e., a gray level of 0) to white (i.e., a gray level of 255).
In the present Yellow pages production, the line artwork on the hard copy master pages is developed by first scanning the original in digital fashion (i.e., element by element). This scanning is at some frequency in elements per inch and results in a digital signal for each elemental area of the original. These digital signals are then subjected to a thresholding process to accommodate the system phototypesetter which is binary and, thus, can only typset black or white graphics and text.
In the thresholding operation, each signal is converted to a gray level indicative of black or white. Usually, a gray level for the threshold is selected halfway between the gray level values for black and white. Thus, the threshold is set at 128 (i.e., one-half of 255) and digital signals indicative of gray levels below 128 are converted to 0 or black gray level signals and those above 128 are converted to 255 or white gray level signals.
The digital signals after thresholding are paginated with the listings and then stored electronically on a page by page basis leaving blank areas for any screened artwork to be included on a page. The latter artwork, in turn, is photographed from the original and the photographs are also stored.
When the hard copy master pages are produced, the electronically stored digital signals are used to address the system phototypesetter which typesets the pages in accordance with the digital signals. The resultant phototypesetted pages also contain the blank areas for the screened artwork. The photographs containing this artwork are then pasted in the corresponding areas to complete production of the hard copy master pages.
As can be appreciated, the need to form the hard copy master pages by pasting on photographs of the screened artwork is tedious, time consuming and costly. Electronically handling the screened artwork, like the line artwork, would, therefore, appear to be a more desirable way to process this artwork. However, attempts at scanning and digitizing the screened artwork (typically, this artwork is at a screen frequency of 85 halftone dots per inch) with the existing scanning and printing equipment for the line artwork (typically operated at a scanning frequency of 723 lines or elements per inch) have proven unsatisfactory.
Thus, the electronically reproduced screened artwork exhibits so called Moire patterns which are beat patterns of the scanning and screen frequencies. These beat patterns are a result of incorrectly reproducing the size of the halftone dots of the screened artwork. The electronically reproduced screened artwork also exhibits incorrect tone which results in low quality of highlight and shadow details. Thus, the existing scanning equipment is incapable of providing a satisfactory digitized reproduction on the hard copy master pages of the screened artwork.
It is, therefore, a primary object of the present invention to provide a system and method which can be used to electronically reproduce screened or halftone dot artwork with substantial elimination of Moire patterns.
It is a further object of the present invention to provide a system and method which can be used to electronically reproduce screened artwork with good tonal quality.